[Edi]

The specialsite terrain masks just mean that in random site determination sites of those paths are favored.

#1 is the way it works. Final terrain mask is the sum of all applicable individual masks.

Assigning sites by #feature and #knownfeature lock that site in stone, i.e. it's always there. The special terrain masks affect the probabilities of available random sites, so you are more likely but not guaranteed to get something.